The human immune response to vaccine antigens such as HBsAg depends on processing to antigenic fragments, binding to MHC antigens, and recognition by the T cell receptor. We have previously described at least 3 pathways of antigen processing for recognition by a panel of human T cell clones. These are: endosomal processing for MHC class II presentation, and nonendosomal processing for MHC class I and class II presentation. The present project is to identify the signals that determine the choice of antigen processing pathway. We are studying two signals found in viruses and in normal cell physiology for their effect on gaining or losing access o each processing pathway. The first signal is the fusion peptide found in HBsAg and several other viral proteins, including gp41 of HIV-1 and influenza hemagglutinin. This signal sequence, FLG, will be mutated to ALG or QLG, and tested for the ability of endogenously expressed antigen to enter all 3 pathways, as observed for the native antigen. If fusion is required to escape from the "normal" pathway for endogenous antigen, nonendosomal processing for class I, then one or more of the two other processing pathways may be lost. The signal will also be tested for its effect on the presentation of exogenous antigen. The second signal is the endoplasmic retention signal, KDEL, that normally retains appropriate enzymes in the ER. In this case, we will add KDEL at the carboxyl end of the antigen and observe its effects on processing. If the endosomal pathway is far from the ER, then ER, then ER retention may block entry into this pathway. Similarly, access to the nonendosomal pathway may also be blocked by this signal. If either signal is critical for the choice of antigen processing pathway, then we may use this to control the immune response. Selective antigen presentation with MHC class II would favor antibody production, while presentation with class I would favor CTL induction. The ability to manipulate the immune response at will would be generally useful for a wide variety of vaccines